Macrophage (M)-tropic HIV-1 quasispecies are the predominant strains replicating in vivo at the time of seroconversion and during the asymptomatic stages of HIV infection. In approximately 40% of HIV- infected individuals, T cell (T)-tropic HIV strains emerge as the predominant species during the course of HIV infection; this ?shift? to T-tropic strain dominance is associated with rapid CD4+ T-cell decline and rapid disease progression. The purpose of this study was to investigate whether the ligands of the beta-chemokine receptor CCR5 (MIP-1alpha, MIP-1beta and RANTES), which block the entry/replication of macrophage (M)-tropic HIV strains in vitro by interfering with the ability of M-tropic HIV to utilize CCR5 as an entry co-receptor, influence the replication of T cell line (T)-tropic HIV strains. Treatment of CD4+ T cells from certain HIV-infected subjects with beta- chemokines in vitro was found to enhance the emergence or replication of endogenous T-tropic HIV quasispecies. The infection efficiency of low inocula of T-tropic HIV in CD4+ T cells from uninfected donors was dramatically increased by treatment of cells with numerous beta- chemokines, including MCP-1, which is not a CCR5 ligand. RANTES was demonstrated to enhance the entry of T-tropic HIV strains into CD4+ T cells and this effect was dependent on Gi protein-coupled signal transduction; in contrast, its antagonist, aminooxypentane (AOP)- RANTES, inhibited M-tropic entry but did not increase T-tropic HIV entry. Enhancement of T-tropic HIV entry by beta-chemokines was associated with increased colocalization of CD4 and the T-tropic coreceptor CXCR4, but not with elevated expression of CXCR4 on CD4+ T cells. These observations suggest that beta-chemokines may play a role in the shift from predominantly M-tropic to T-tropic HIV strain replication in vivo and that the administration of beta-chemokines to HIV-infected subjects for the purpose of limiting the replication and spread of HIV should be approached with caution as such treatment may result in the accelerated emergence of T-tropic HIV strains. - HIV; T cells; chemokines; T-tropic HIV strain; signal transduction